On Thu, Aug 26, 2004 at 01:11:52AM +0100, Jamie Lokier wrote:
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> Is this a problem if we treat entering a file-as-directory as crossing
> a mount point (i.e. like auto-mounting)?

Yes - mountpoints can't be e.g. unlinked.  Moreover, having directory
mounted on non-directory is also an interesting situation.

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> Simply doing a path walk would lock the file and then cross the mount
> point to a directory.

*Ugh*

What would happen if you open that directory or chdir there?  If it's
"underlying file stays locked" - we are in even more obvious deadlocks.

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> A way to ensure that preserves the lock order is to require that the
> metadata is in a different filesystem to its file (i.e. not crossing a
> bind mount to the same filesystem).
> 
> That has the side effect of preventing hard links between metadata
> files and non-metadata, which in my opinion is fine.

We don't actually need a different fs - different vfsmount will do just fine.

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> The strict order is ensured by preventing bind mounts which create a
> path cycle containing a file->metadata edge.  One way to ensure that
> is to prevent mounts on the metadata filesystems, but the rule doesn't
> have to be that strict.  This condition only needs to be checked in
> the mount() syscall.

You really don't want to lock mountpoint on path lookup, so I don't see
how that would be relevant - it's a hell to clean up, for one thing
(I've crossed ten mountpoints on the way, when do I unlock them and
how do I prevent deadlocks from that?)  Besides, different namespaces
can have completely different mount trees, so tracking down all that
stuff would be hell in its own right.

The main issue I see with all schemes in that direction (and something
like that could be made workable) is the semantics of unlink() on
mountpoints.  *Especially* with users being able to see attributes of
files they do not own (e.g. reiser4 mode/uid/gid stuff).  Ability to
pin down any damn file on the system and make it impossible to replace
is not something you want to give to any user.